Technical Field
The present invention relates to an electric drive vehicle mounted with a voltage converter.
Related Art
In an electric drive vehicle such as a hybrid vehicle and an electric vehicle which uses a rotary electric machine as a driving source, DC power of a battery is converted into AC power by an inverter to drive the rotary electric machine. A buck-boost converter is further provided between the battery and the inverter to boost the battery voltage and step down the regenerative power from the rotary electric machine. As a converter with extended functions of such a buck-boost converter, for example, JP 2012-70514 discloses a voltage converter which is provided with four switching elements and connected to two batteries. With this voltage converter, boosting and stepping down of voltage become possible by turning the switching elements ON and OFF. It also becomes possible to switch between a series connection and a parallel connection of the two batteries by alternating the ON/OFF pattern of the switching elements.
In a series connection, because the output voltage is equal to the sum of the voltages of the two batteries, it is possible to lower the voltage boosting rate of each battery to a relatively low rate. Therefore, an advantage can be achieved that loss in the switching elements to be used for boosting the voltage is relatively low. However, in the series connection, it is disadvantageous that the output current is limited to that of the battery which has a lower tolerable electric current (maximum current) of the two batteries.
In a parallel connection, it is problematic that as the voltage boosting rate of each battery is higher than in the series connection, the loss in the switching elements is increased. However, as no current limit is applied, relatively large output of current becomes possible. In other words, with the same voltage, a larger electric power can be output with the parallel connection than with the series connection.